That's because of different expectations. For a tape drive, you don't care about latency, power efficiency, random access or write amplification, all that matters is getting as much data as possible on a tape for the lowest amount of investment into tapes. So the tapes are advertised with the amount of net data that you can fit onto it, assuming a standard distribution of compressible vs non-compressible data.
For a laptop SSD, all of these matter. You can't do much compression because compression consumes a lot of power and latency upon access, some compression schemes make random access harder (i.e. compression schemes without an index where you have to scan through intermediate checkpoints or, in the worst case, sequentially through the entire media), and it may lead to write amplification as well if you need to add a piece of data in the middle of a file (basically the issue with shingle HDDs). As a result, no compression possible, and so SSDs are advertised with the raw capacity (or, in fact, they are underadvertised because SSDs need spare block capacity to account for wear).
But it completely disregards that most files you'd want to backup are already compressed! Even if I where to turn on aggressive compression for my ssd despite all the inconveniences, it wouldn't change much. I'll have to agree with the op, storage capacity marketing for tape is weird if not outright a scam.
A lot of tape usage is in mainframe and traditional big iron systems where data is bulk unloaded from databases in a fairly raw format, and it’s convenient to let the tape infrastructure handle both compression and encryption.
I had a lto4 (max capacity 800gb) last year that took over 5tb of data. The data was ASCII log files from a server that had been running for years. Lots of repetition (time stamps, everything ok messages, you have mail messages) and the drive compression ate it for breakfast.
For a laptop SSD, all of these matter. You can't do much compression because compression consumes a lot of power and latency upon access, some compression schemes make random access harder (i.e. compression schemes without an index where you have to scan through intermediate checkpoints or, in the worst case, sequentially through the entire media), and it may lead to write amplification as well if you need to add a piece of data in the middle of a file (basically the issue with shingle HDDs). As a result, no compression possible, and so SSDs are advertised with the raw capacity (or, in fact, they are underadvertised because SSDs need spare block capacity to account for wear).